Striated muscle cells consist of three sets of filaments: (1) thick, myosin containing filaments, which interdigitate with (2) thin, actin- containing filaments and (3) titin filaments, which center the thick filaments axially in the sarcomere and produce the resting tension. The resting tension is different in fibers taken from different muscles: the tension produced by individual rabbit soleus muscle fibers is about five times smaller than that produced by rabbit psoas muscle fibers. The influence of sarcomere length on the movement of thick filaments away from the center of the sarcomere, which depends on the elastic properties of the titin filaments, shows that the titin filaments are stiffer in psoas fibers than in soleus fibers. Therefore the different tension levels in soleus and psoas fibers are due to quantitative difference in the elastic properties of the titin molecules in the different muscle fibers. The discovery that the weakly-binding actomyosin crossbridge has a different structure than the strongly binding crossbridge raised the possibility that physiological contraction is driven by a transition between these two states. This does not appear to be the case, however, because the force developed when nonhydrolyzable ATP analogues are first added to and then removed from covalently crosslinked muscle fibers depends on the level of preapplied stretch: no force is produced if the fiber is not prestretched. When the same muscle fiber is activated by calcium in an ATP-containing solution, larger forces are produced which are independent of the prestretch level (up to 1.5% muscle length). Thus the weak-strong crossbridge transition is not likely to be the physiological force-generating mechanism.